1. Field of the Invention
This invention relates generally to apparatus and circuits for starting single phase motors, and more particularly to a starting device and circuit for starting single phase motors utilizing auxiliary capacitance provided by a pre-assembled universal add-on start kit connected across the start and run winding of an induction motor or in parallel with the run capacitor of a permanent split capacitor motor with only two interchangeable non-polarized wires.
2. Brief Description of the Prior Art
Single phases motors are commonly classified as split phase motors, permanent split capacitor motors, capacitor start-capacitor run motors and capacitor start-induction run motors. Most single phase induction motors require some type of switching arrangement for starting the motor, usually by switching start windings, a start capacitor, a run capacitor or a combination thereof until the motor reaches full speed. Capacitor start motors require a start capacitor only during the starting period of the motor. A permanent split capacitor motor has a run capacitor connected permanently across the run winding and start winding terminals of the motor.
It is often necessary to add auxiliary capacitance to the run capacitor used in conjunction with a permanent split capacitor motor (referred to as a PSC motor) during the starting phase of the motor so as to maintain the power factor as near to one as possible in order to achieve maximum power input and therefore apply maximum torque to the rotor of the PSC motor until the rotor reaches full speed. This starting phase usually lasts less then one second and the auxiliary capacitor must be disengaged from the circuit near or at the time the rotor reaches full speed.
Several auxiliary starting devices have been utilized to add auxiliary capacitance to the run capacitor of these type of motors. These auxiliary starting devices are commonly referred to as "Hard-Start Kit", "Start Assist" or "Motor Torque Multiplier". The two most common devices employed are:
One common auxiliary starting device utilizes an auxiliary capacitor in series with a solid state material which rapidly increases in resistance as it is heated when an electric current passes through it, thereby effectively disconnecting the auxiliary capacitor from the circuit soon after power is applied to the motor. However, one of the major problems with this method is that the solid state material remains hot from the "trickle current" that continues to pass through it as long as power is applied to the motor. Once the power has been disconnected from the motor, the solid state material begins to cool which requires approximately one to two minutes. If power is re-applied during the cooling off period, the auxiliary capacitor is ineffective because it is still de-energized.
Another auxiliary starting device utilizes an auxiliary capacitor electrically connected with two wires in parallel to the run capacitor through a normally closed set of contacts of an electromechanical relay commonly known as a potential relay. The coil of the relay is connected in parallel with the start winding of a permanent split capacitor (PSC) motor. As power is applied to the motor, the voltage across the start winding rises and when that voltage reaches a threshold near full speed of the rotor of the motor, the potential relay energizes, the normally closed contacts open and disconnect the auxiliary capacitor from the circuit. The potential relay remains energized until the power to the motor is disconnected. Once the power is disconnected, the circuit returns to the power-off state and the motor can be restarted immediately.
One of the major problems with this circuitry is that it requires three electrically conducting wires, one connected to the common terminal, one to the run terminal, and one to the start terminal of the motor. These wire connections are polarized, which means that each wire must be connected to a dedicated terminal and the wire connections can not be interchanged. Serious damage and injuries often occur because the wires have been connected incorrectly. This problem occurs so often that some air conditioning manufacturers have factory installed a plug for receiving auxiliary capacitance devices to prevent damage to their equipment caused by field service personnel incorrectly connecting the wires.
Another problem with this method is that the threshold voltages across the start windings vary greatly for different motor manufacturers and different motor sizes, and a variety of potential relays with different coil voltage energizing ratings must be used within a power voltage range (110 volts, 230 volts, etc). This means that a service man must always have available a very wide range of relay and capacitor combinations. For example, approximately ten different potential relays, each with a different coil voltage rating are available for 230 volt PSC motors alone. The above described auxiliary capacitance starting method requires field assembly and connecting the field assembled components to the dedicated terminals with three polarized wires. An arrangement which is time consuming, costly and subject to making wire connection errors.
There are also several patents which disclose various apparatus and methods of starting single phase motors.
Blaha, U.S. Pat. No. 4,012,678 discloses a wiring arrangement to minimize contact welding of a potential relay employed in a starting circuit. It does so by switching resistors and capacitors in series with the windings of an induction motor.
Robinson, U.S. Pat. No. 3,454,858 discloses a "hot-wire" thermal starting relay used in conjunction with a specially designed capacitor for fractional horse power motors.
Enemark et al, U.S. Pat. No. 3,521,138 discloses a thermally responsive bimetal starting mechanism for single phase motors. As pointed out above, thermally responsive devices, whether they are hot wire, bimetal, use a thermistor or ceramic material can not be restarted immediately once the power has been removed, because the aforementioned materials must cool to ambient temperatures before they become operative.
Enemark, U.S. Pat. No. 3,538,409 discloses a specially designed capacitor which when heated from current passing through it changes capacitance and therefore changes from a higher torque to a low torque starting system.
Hildebrandt, U.S. Pat. No. 4,658,196 discloses mechanical governor switches connected to the shaft of a motor which de-energizes and disconnects a start capacitor from the circuit when the motor shaft reaches full speed.
The present invention is distinguished over the prior art in general, and these patents in particular by an auxiliary capacitance starting device which is connected by only two wires to a permanent split capacitor motor. The device includes an auxiliary capacitor and a potential relay which has a set of normally closed contacts and a relay coil. The auxiliary capacitor and the normally closed contacts are connected in series and the relay coil is connected in parallel therewith. The free ends of the two non-polarized wire leads are interchangeably electrically connected to the start and run windings or the run capacitor of the motor to place the circuit in parallel with the start and run windings or the run capacitor such that the relay coil means becomes energized upon detection of voltage representing full motor speed to open the contacts and break the electrical connection between the auxiliary capacitor and the start and run windings. The relay coil becomes de-energized upon detection of no voltage to close the contacts to allow the immediate re-application of electrical power to the common and run terminals of the motor. The components and circuitry are enclosed in a generally rectangular housing which may be mounted on a suitable surface in close proximity to the motor.